This is completely free, and is a great audio and video converter. I have used it in the past to convert between wma, wav and mp3. It will let you choose the quality of the mp3 you are converting to also.
http://www.erightsoft.net/S6Kg1.html
One thing I must warn you about tho. When you first open the programme, click on the main screen where the files to be converted are placed. Make sure you choose an appropraite folder for the converted files to be put. Just create a folder on the desktop called 'converted music' and make sure you tell it to send to there.